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The Structure of the Earth and Plate Tectonics The History of Plate Tectonics • Alfred Wegener is considered by most scientists to be the pioneer of the continental drift theory. • Wegener proposed that the continents were once connected and shared common species (Pangaea) • Later these continents drifted apart carrying their inhabitants with them. The History of Plate Tectonics • Wegener proposed that all of today’s land masses were once connected Pangaea (200 mya) The History of Plate Tectonics The History of Plate Tectonics Evidence • Using the fossil record, you can piece the continents together like a puzzle Distribution of fossils The History of Plate Tectonics Evidence • Wegener showed that the same three layers occur at several locations on the earth • The middle layer is composed of sandstone, shale, and coal beds Harry Hess and J Wilson propose a mechanism to explain continental movement in the early 60’s. • In 1969 the Glomar Challenger drilled a series of holes in the Mid-Atlantic ridge. What they found changed geology and oceanography. • Since the ocean floor is lower in the lithosphere, scientists expected to find older rocks at those depths. -Continental geology’s law of Superposition states that oldest rocks are laid down first and should be found horizontally lowest in a bed unless uplifted. • They found no rock older than 3 billion years and most were younger. How could the ocean floor be younger than the continents riding on it? The Deep Sea Drilling project showed that rocks became older as they moved away from the mid ocean ridge. Oceanic geology showed rock layers are created vertically. Plate Tectonics • The Earth’s crust is divided into 7-8 major plates which are moved in various directions. – The word, tectonic, refers to the deformation of the crust as a consequence of plate interaction. • This plate motion causes them to collide, pull apart, or scrape against each other. • Each type of interaction causes a characteristic set of Earth structures or “tectonic” features. What are tectonic plates made of? • Plates are made of rigid lithosphere. The lithosphere is made up of the crust and the upper part of the mantle. What lies beneath the tectonic plates? • Below the lithosphere (which makes up the tectonic plates) is the asthenosphere. Plate Movement • “Plates” of lithosphere are moved around by the underlying hot mantle convection cells What happens at tectonic plate boundaries? Three types of plate boundary • Divergent • Convergent • Transform Divergent Boundaries • Spreading ridges – As plates move apart new material is erupted to fill the gap. 1. The picture to the left is of Iceland where the Mid-oceanic ridge comes to the surface for us to see. 2. The geographic feature created from divergent plates is a ridge system Age of Oceanic Crust Courtesy of www.ngdc.noaa.gov Convergent Boundaries • There are three styles of convergent plate boundaries – Continent-continent collision – Continent-oceanic crust collision – Ocean-ocean collision Continent-Continent Collision • Forms mountains, e.g. European Alps, Himalayas Continent-Oceanic Crust Collision • Called SUBDUCTION Subduction • Oceanic lithosphere subducts underneath the continental lithosphere • Oceanic lithosphere heats and dehydrates as it subsides • The melt rises, forming volcanoes • E.g. The Andes Ocean-Ocean Plate Collision • When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone. • The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench. • The worlds deepest parts of the ocean are found along trenches. – E.g. The Mariana Trench is 11 km deep! Transform Boundaries • Where plates slide past each other Above: View of the San Andreas transform fault 1. San Andres Fault in California where the boundary of two plates come to the surface! Volcanoes and Plate Tectonics… …what’s the connection? Volcanoes are formed by: - Subduction - Rifting - Hotspots Pacific Ring of Fire Hotspot volcanoes What are Hotspot Volcanoes? • Hot mantle plumes breaching the surface in the middle of a tectonic plate The Hawaiian island chain are examples of hotspot volcanoes. Photo: Tom Pfeiffer / www.volcanodiscovery.com The tectonic plate moves over a fixed hotspot forming a chain of volcanoes. The volcanoes get younger from one end to the other. The Mechanisms Supporting Continental Drift Volcanic Islands Mid-Ocean Ridges Volcanic Mountain Ranges